As disclosed in specifications of U.S. Pat. Nos. 4,699,174 and 4,550,597, steering apparatuses such as power steerings provided to vehicles such as automobiles assist rotating operation power using a power source such as a motor or a hydraulic mechanism at the time of the rotating operation of a steering wheel. A torsion bar of the present invention is unitized in order to control the operation of the power source.
The torsion bar is used on a path for transmitting a rotating power of the steering wheel to a steering gear box. When the steering wheel is rotated, the torsion bar is twisted, so that a difference is generated between a rotation phase on the side of the steering wheel and a rotation phase on the side of the steering gear box. The phase difference is suitably detected so that the operation of the power source is controlled.
In the torsion bar for the steering apparatus, both ends in a longitudinal direction are joint portions which are supported by an object to be attached, and a midway portion in the longitudinal direction is a spring portion which allows torsion. Such a kind of torsion bars include a torsion bar in which its entire shape is a round bar, for example, joint portions on both ends in its longitudinal direction have large diameter, and a spring portion on a midway portion in the longitudinal direction has a small diameter.
The torsion bar used for the above application requires predetermined fatigue strength against repeated torsion. The fatigue strength against repeated torsion is determined so as to satisfy a condition such that when a torsion stress of 300 MPa or more is applied, torsion can be carried out at least 500,000 times.
A method of manufacturing the prior torsion bar is explained. A round bar steel material with low hardness first undergoes a cutting work so as to have an outer shape which is close to a finished product. The steel material having the outer shape close to the finished product by the cutting undergoes high-temperature heat treatment so as to have predetermined high hardness. Finally, warping of a matrix due to the heat treatment is corrected and distortion is removed, so that an outer diameter is adjusted. In such a manner, the prior torsion bar is first shaped into the outer shape close to the finished product of the torsion bar, and then its hardness is adjusted so as to be high.
A main object of the present invention is to provide a method of manufacturing a torsion bar for first giving treatment which heighten a hardness of an entire round bar steel material according to cold drawing work, and working the round bar steel material into an outer shape close to a finished product of a torsion bar, so as not to require high-temperature heat treatment for heightening the hardness.